The present invention relates to a valve gear of an engine for vehicles or the like, and particularly to a valve gear in which cams operative to control opening/closing of a valve are switchable.
A valve gear of an engine, in which plural cams having different-shaped nose portions are provided for each valve, and the valve-opening amount, the valve opening-closing timing, and the like are configured to be changeable according to an engine's operation state through a selection of a specified cam for opening/closing the valve from the plural cams, is known.
Japanese Patent Laid-Open Publication No. 2013-083202 and US Patent Application Publication No. 2011/0226205 A1, for example, disclose that a valve gear, in which a camshaft is comprised of a shaft portion and a cylindrical cam element portion which is coupled to the shaft portion with spline coupling so as to be moved in an axial direction of the shaft portion, the cam element portion has, at its outer periphery, plural cams for each valve which have different-shaped nose portions provided adjacently to each other, and a cam for opening/closing the valve is configured to be switchable through a move of the cam element portion in the axial direction.
Herein, in the valve gear disclosed in the above-described patent documents, a pair of end-face cams are provided symmetrically at both end faces of the cam element portion and there are further provided a pair of operational members, each of which is configured to project to a position facing the corresponding end-face cam and contact this end-face cam so as to move the cam element portion, in the axial direction, toward an arrangement side of the other operational member or retreat from the above-described position facing the corresponding end-face cam. The above-described operational members are driven (projected) by actuators, so that switching operation of the cams can be conducted.
Meanwhile, it has been recently desired for the engine equipped with the above-described valve gear that the switching to the best cam is conducted in every combustion cycle in accordance with the engine's driving state, that is—that the cam switching is conducted continuously in a moment. While it is necessary to drive the actuator so that the operational member can project or retreat at a desired timing in order to fulfill the above-described desire, it may be difficult that no malfunction happens to the operational member. Herein, the end-face cams provided at the both sides of the cam element portion are provided symmetrically such that respective maximum-lift portions thereof are positioned at the same phase in the valve gear disclosed in the former of the above-described patent documents. Therefore, when the cam portion is switched by making one of the operational members project, thereby moving the cam element portion toward the arrangement side of the other operational member, if the other operational member projects erroneously because of the operational malfunction or the like, there is a phase in which the length between the end-face cams provided at the both sides of the cam element portion is greater than an arrangement distance between the pair of operational members. As shown in FIG. 12, at a specified phase in which the length between both-side end-face cams 123, 123 is greater than the arrangement distance Lpin between two operational members 132, 132, a cam element portion 120 gets stuck between the both-side operational members 132, 132, so that there is a concern that a camshaft 102 may lock and stop rotating.